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Commit d7b20279 authored by Zheng Yejian's avatar Zheng Yejian Committed by Greg Kroah-Hartman
Browse files

ring-buffer: Fix wrong stat of cpu_buffer->read 

[ Upstream commit 2d093282b0d4357373497f65db6a05eb0c28b7c8 ]

When pages are removed in rb_remove_pages(), 'cpu_buffer->read' is set
to 0 in order to make sure any read iterators reset themselves. However,
this will mess 'entries' stating, see following steps:

  # cd /sys/kernel/tracing/
  # 1. Enlarge ring buffer prepare for later reducing:
  # echo 20 > per_cpu/cpu0/buffer_size_kb
  # 2. Write a log into ring buffer of cpu0:
  # taskset -c 0 echo "hello1" > trace_marker
  # 3. Read the log:
  # cat per_cpu/cpu0/trace_pipe
       <...>-332     [000] .....    62.406844: tracing_mark_write: hello1
  # 4. Stop reading and see the stats, now 0 entries, and 1 event readed:
  # cat per_cpu/cpu0/stats
   entries: 0
   [...]
   read events: 1
  # 5. Reduce the ring buffer
  # echo 7 > per_cpu/cpu0/buffer_size_kb
  # 6. Now entries became unexpected 1 because actually no entries!!!
  # cat per_cpu/cpu0/stats
   entries: 1
   [...]
   read events: 0

To fix it, introduce 'page_removed' field to count total removed pages
since last reset, then use it to let read iterators reset themselves
instead of changing the 'read' pointer.

Link: https://lore.kernel.org/linux-trace-kernel/20230724054040.3489499-1-zhengyejian1@huawei.com



Cc: <mhiramat@kernel.org>
Cc: <vnagarnaik@google.com>
Fixes: 83f40318 ("ring-buffer: Make removal of ring buffer pages atomic")
Signed-off-by: default avatarZheng Yejian <zhengyejian1@huawei.com>
Signed-off-by: default avatarSteven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
parent f6e1e569

kernel/trace/ring_buffer.c

+12 −10
Original line number Diff line number Diff line
@@ -485,6 +485,8 @@ struct ring_buffer_per_cpu { 
	unsigned long			read_bytes;

	u64				write_stamp;

	u64				read_stamp;

	/* pages removed since last reset */

	unsigned long			pages_removed;

	/* ring buffer pages to update, > 0 to add, < 0 to remove */

	long				nr_pages_to_update;

	struct list_head		new_pages; /* new pages to add */
@@ -520,6 +522,7 @@ struct ring_buffer_iter { 
	struct buffer_page		*head_page;

	struct buffer_page		*cache_reader_page;

	unsigned long			cache_read;

	unsigned long			cache_pages_removed;

	u64				read_stamp;

};
@@ -1581,6 +1584,8 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages) 
		to_remove = rb_list_head(to_remove)->next;

		head_bit |= (unsigned long)to_remove & RB_PAGE_HEAD;

	}

	/* Read iterators need to reset themselves when some pages removed */

	cpu_buffer->pages_removed += nr_removed;



	next_page = rb_list_head(to_remove)->next;
@@ -1602,12 +1607,6 @@ rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned long nr_pages) 
		cpu_buffer->head_page = list_entry(next_page,

						struct buffer_page, list);



	/*

	 * change read pointer to make sure any read iterators reset

	 * themselves

	 */

	cpu_buffer->read = 0;



	/* pages are removed, resume tracing and then free the pages */

	atomic_dec(&cpu_buffer->record_disabled);

	raw_spin_unlock_irq(&cpu_buffer->reader_lock);
@@ -3659,6 +3658,7 @@ static void rb_iter_reset(struct ring_buffer_iter *iter) 


	iter->cache_reader_page = iter->head_page;

	iter->cache_read = cpu_buffer->read;

	iter->cache_pages_removed = cpu_buffer->pages_removed;



	if (iter->head)

		iter->read_stamp = cpu_buffer->read_stamp;
@@ -4101,12 +4101,13 @@ rb_iter_peek(struct ring_buffer_iter *iter, u64 *ts) 
	buffer = cpu_buffer->buffer;



	/*

	 * Check if someone performed a consuming read to

	 * the buffer. A consuming read invalidates the iterator

	 * and we need to reset the iterator in this case.

	 * Check if someone performed a consuming read to the buffer

	 * or removed some pages from the buffer. In these cases,

	 * iterator was invalidated and we need to reset it.

	 */

	if (unlikely(iter->cache_read != cpu_buffer->read ||

		     iter->cache_reader_page != cpu_buffer->reader_page))

		     iter->cache_reader_page != cpu_buffer->reader_page ||

		     iter->cache_pages_removed != cpu_buffer->pages_removed))

		rb_iter_reset(iter);



 again:
@@ -4538,6 +4539,7 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer) 
	cpu_buffer->last_overrun = 0;



	rb_head_page_activate(cpu_buffer);

	cpu_buffer->pages_removed = 0;

}



/**